Field of the Invention
The present invention relates to an insert for placement in the tubes of a radiant tube furnace to increase heat transfer to the material being heated and to improve the fuel efficiency of the furnace. More particularly, it relates to a radiant tube insert that transfers more heat to the portion of the radiant tube that is closest to the material being heated.
Description of the Related Art
Radiant tube combustion furnaces are commonly used to heat materials such as ferrous and non-ferrous metals including steel and aluminum. Such radiant tube furnaces may be continuous furnaces where the material being heated is continuously passed through the furnace or may be batch furnaces where a large load of material is placed in the furnace. As an example a radiant tube continuous combustion furnace 10, as illustrated in FIGS. 1 and 2, is a generally rectangular box-shaped structure having a roof 11, a floor 13, and two side walls 15. Two sets of radiant tubes 12 are located in the furnace, an upper set that is closer to the roof and a lower set that is closer to the floor. The material 14 being heated is passed between the upper set of radiant tubes 12 and the lower set of radiant tubes 12. Each radiant tube 12 has a burner section 16 attached to a combustion burner 17 and an exhaust section 18 through which the combustion gases, commonly referred to as flue gas, exit the radiant tube 12. While FIGS. 1 and 2 show a U-shaped radiant tube, the radiant tube may take other suitable shapes including straight and W-shaped. Heat generated by the combustion burner 17 is transferred through the walls of the radiant tube 12 to the material 14 being heated. In the burner section 16, thermal energy from the inside of the radiant tube 12 can be transmitted from the radiant tube 12 through convection from the high temperature combusted gas passing through the tube 12 and, near the burner end, by radiation from the bright combustion flame. In the exhaust section 18, thermal energy can only be transmitted through convection from the remaining combustion gas passing through the tube 12. Further, the available heat in the exhaust section 18 is lower because the combustion gas loses energy while traveling down the radiant tube 12, as indicated by directional arrows 20, causing the combustion gas at the exhaust end of the tube 12 to be at a significantly lower temperature than at the burner end.
Because of this configuration, more thermal energy is transmitted from the burner section 16 relative to the exhaust section 18. This creates uneven heat transfer to the material 14 that is being heated, which, in this case, is travelling in a direction perpendicular to the radiant tubes 12 as indicated by arrows 22 and, in the case of a batch furnace, is stationary. A large amount of thermal energy is also wasted in the exhaust section 18, as most of the thermal energy exits the furnace 10 without any means to direct it to the material 14 being heated.
Inserts that can be arranged inside of the exhaust section 18 have been previously created in order to increase the overall heat transfer to the material 14 being heated, as well as, to more evenly distribute the amount of energy given off by the burner section 16 and the exhaust section 18. This is accomplished by mixing and forcing more exhaust gas to the interior surface 24 of the radiant tube 12, as well as by transmitting radiant energy that the insert collects. These designs have been proven to increase furnace efficiency by 5-20%, which reduces costs of continuous furnace operation.